TheHarbor Hill Moraine, in thegeography of Long Island, forms the northern of two ridges along the "backbone" ofLong Island.[1]
The Harbor Hill Moraine, skirting theNorth Shore, represents theterminal moraine of the most recent advance of theWisconsinian glaciation, which reached its most southward advance about 18,000 years ago; the earlierRonkonkoma Moraine, much cut through by outwash streams from the Harbor Hill Moraine, lies to the southeast. The Harbor Hill moraine is represented by theNorth Fork of eastern Long Island and in three disjunct sections farther east,Plum Island,Great Gull Island, andFisher's Island. The western end is theHeights of Guwan in Brooklyn.
The Harbor Hill Moraine, named for its prominence atHarbor Hill,Roslyn, New York, the highest point inNassau County, resulted from a lingering equilibrium stage in the glacier's episodic retreat, creating a stationarymelting front;[2] the Long Island area became permanently free of glacial ice in the range of 13,000 to 12,000 years ago.[3]
The Harbor Hill Moraine intersects with the earlier Ronkonkoma Moraine atLake Success. East of the lake, the two moraines may be seen, but west of Lake Success the Harbor Hill moraine overrode and effaced the Ronkonkoma moraine.[4] South of the moraines is a broadoutwash plain sloping towards theAtlantic Ocean, much of which has been submerged as ocean levels rose in the post-glacial period. The Harbor Hill Moraine forms the major drainage divide inNassau County. Streams to the northwest of the moraine flow intoLong Island Sound, whereas those to the south empty into the series of bays orsounds behind thebarrier islands and so into the Atlantic.
